Method and apparatus for defeating the effects of a video anti-copy process

Abstract: A known video anti-copy process adds pseudo-sync and AGC pulses into the vertical blanking interval of a video signal when a copy of the video signal is made, the presence of the copy protect pulses causing the automatic gain control system in the videotape recorder to make a copy with an abnormally low amplitude video signal. The apparatus and method of the invention is arranged to defeat this known anti-copy process by reducing the effectiveness of the copy protect pulses, rather than by removing them. In the method of the invention, other pulses are added to the video signal to counteract the gain reduction caused by the copy protect pulses. Specifically, the back porch levels are moved from blanking to below blanking level.

Description: The present invention relates to a method and apparatus for defeating the effects of a video anti-copy process that adds pulses to blanking intervals of a video signal. US-A-4,631,603 describes a video anti-copy process in which a video signal is modified so that a television receiver will still provide a normal color picture from the modified video signal while the video tape recording of the modified video signal produces generally unacceptable pictures. The anti-copy protection relies on the fact that typical videocassette recorder automatic gain control systems cannot distinguish between the normal sync pulses (including equalizing or broad pulses) of a conventional video signal and added pseudo-sync pulses. Pseudo-sync pulses are defined here as any other pulses which extend down to normal sync tip level and which have a duration of at least 0.5 microseconds. A plurality of such pseudo-sync pulses are added to the conventional video signal during the vertical blanking interval, and each of said pseudo-sync pulses is followed by a positive pulse of suitable amplitude and duration. As a result, the automatic gain control system in a videotape recorder will make a false measurement of video level which causes an improper recording of the video signal. The result is unacceptable picture quality during playback. Thus, this basic anticopy process, described in the prior art, causes an abnormally low amplitude video signal to be recorded when a copy is attempted. Some of the effects observed when the illegal copy is replayed are horizontal tearing (positional displacement) and vertical displacement of the picture. "Macrovision Decoder/Blanker", Elektor Electronics, Vol. 14, No. 160, October 1988, pages 44-47, XP000046163 describes an apparatus for defeating an anti-copy process in which pulses are added to blanking intervals of a video signal. The apparatus removes some, but not all, of the added pulses and replaces them with a continuous black level. With this proposal it is not possible to blank out all of the anti-copy pulses as this gives rise to color purity errors, and so the resultant video signal may still be capable of interfering with the recording process by a videotape recorder. The present invention seeks to provide a new method and apparatus for defeating the prior art anti-copy process. According to a first aspect of the present invention there is provided a method of defeating the effects of a video anti-copy process which adds pulses to blanking intervals of a video signal, the method comprising the step of: replacing a particular portion of the video signal with a signal having an amplitude sufficiently below the blanking level of the video signal to enable a VTR's AGC amplifier to offset the gain reduction caused by said anti-copy process whereby the anti-copy process is defeated. Preferably, the particular portion comprises a backporch portion of the video signal and has an amplitude at the blanking level. For example, the amplitude below the blanking level is about -20 IRE units. In an embodiment, the method further comprises the steps of generating a plurality of negative-going pulses; and adding the plurality of negative-going pulses to about the last ten active lines of at least some fields of the video signal. The present invention also extends to apparatus for defeating the effects of a video copy protection process which adds pulses to blanking intervals of a video signal, the apparatus comprising: a generator for generating a signal having an amplitude sufficiently below the blanking level of the video signal to enable a VTR's AGC amplifier to offset the gain reduction caused by said anti-copy process; and means for replacing a particular portion of the video signal with said generated signal having the amplitude sufficiently below the blanking level whereby the anti-copy process is defeated. In an embodiment, said replacing means comprises: means for determining a back porch portion of the video signal, the back porch having an amplitude at the blanking level of the video signal; and means for replacing the back porch portion with said generated signal having an amplitude below the blanking level. For example, the apparatus may further comprise a generator for generating a plurality of negative-going pulses; and means for adding the plurality of negative-going pulses to about the last ten lines of at least some fields of the video signal. Embodiments of the present invention will hereinafter be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a block diagram of an apparatus, which is not an embodiment of the invention, for removing various video signal modifications; Figures 2, 3 and 4 show circuits for removing anticopy process enhancement signals via level shifting and horizontal sync replacement; Figure 5 shows a circuit for removing anticopy process enhancement signals via new sync and burst position replacement; Figure 6 shows a circuit for removing anticopy process enhancement signals via multiplying; Figures 7, 8 and 9 show circuits for removing anticopy process enhancement signals via switching means; Figure 10 shows a circuit for defeating enhancement signals via DC averaging and attenuation; Figures 11a and 11b show waveforms illustrating the defeat of enhancement signals via increasing sync amplitude; Figure 12 shows a circuit for defeat of enhancement signals via increasing sync amplitude; Figure 13 shows a circuit for defeat of enhancement signals via tracking and holding circuits; Figures 14a and 14b show circuits for defeat of the basic anti-copy process in accordance with the present invention, and Figures 15a to 15g show waveforms illustrating operation of the circuits of Figures 14a, 14b. There is described herein a method and apparatus for defeating the effects of a video anti-copy process which adds pulses to blanking intervals of a video signal. For example, in the methods described in US Patent No. 4,631,603 AGC pulses and pseudo-sync pulses are added to blanking intervals to provide copy protection. US Patents Nos. 4,695,901 and 5,157,510 each disclose methods and apparatus for defeating (removal of attenuation of) these added pulses. For example, US Patent No. 4,695,901 discloses the removal or attenuation of pseudo-sync and AGC pulses, but does not disclose the defeat of copy protection arising from sync pulse narrowing, end of field pseudo vertical sync pulses, or end of line (checker) pulses which are all techniques described in the parent specification EP-A-0706740. Processing amplifiers as are well known in the art can remove sync pulse narrowing with regenerated sync, but processing amplifiers do not defeat the end of line checker pulses or end of field pseudo-vertical sync pulses. It is not taught in the prior art how to defeat these copy protection vertical and checker signals. Merely blanking them out can still cause residual anticopy signal enhancements to survive when an illegal copy is made. The reason is that blanking these signals to blanking level alone in the presence of pseudo-sync and AGC pulses will cause an attenuated video signal to be input into a TV set when an illegal copy is made. This attenuated signal then has, for instance, end of field lines at blanking level and can cause pseudo-vertical sync pulses in this situation. This is true especially if narrowed horizontal sync pulses are still present. On the other hand, if only the narrowed sync pulses are restored to normal sync width, the other two copy protection enhancement signals (checker and vertical) are still effective. Although the present invention is concerned with defeating the effects of pulses added to blanking intervals to provide copy protection, it is helpful to consider other copy protection techniques and their manner of defeat, and a number of such techniques and methods for their defeat are described below. The following methods may be used to defeat the copy protection techniques outlined above: 1. End of line (checker modification) copy protection signals may be replaced with a signal at least about 20% of peak white, or a level shifting signal of at least 20% of peak white may be added to the end of line signal. The signal replacement or adding may be to a portion of the checker signal so as to defeat the process. By "portion" is meant the part of the end of line pulse to be "neutralized" or part of all the lines of video that has the end of line pulses. 2. End of field (vertical) copy protection pulses may be replaced with a signal that is at least about 20% of peak white for a period of at least around 32 µsec per line. Alternatively, a level shifting signal of at least about 20% of peak white may be added to the vertical pulses for at least around 32 µsec on enough lines (i.e. 7 of 9, 5 of 7, 2 of 3) to defeat the anticopy process. The 20% of peak white level referred to here with respect to the vertical and checker pulses has been experimentally determined to be a typical minimum value needed to produce the intended effect of defeating the video signal enhancements. A higher level signal (such as 30% or more) would accomplish the purpose even more thoroughly. 3. Most (50% or more) of narrowed horizontal sync pulses are widened so as to defeat the sync narrowing process, i.e. if sync is narrowed to 3.0 µsec, a widened sync pulse to 4.0 µsec may be adequate to defeat the sync narrowing process, and without the need to replace the narrowed sync with RS170 standard horizontal sync pulses. (Note that 4.7 µsec is specified as the horizontal sync pulse width for the RS170 standard). 4. A widened sync pulse width encroaching into the end of line (checker) pulses may be used to defeat both the checker and sync narrowing processes. Care must take to assure that the flyback pulse of the TV set still triggers color burst, because widening the sync pulse to include part of the checker pulses can cause the TV set's flyback pulse to trigger prematurely. 5. A horizontal sync and colorburst with adequate sync and burst width repositioned into the checker pulses can defeat both the sync narrowing and the checker processes, without causing the TV set's flyback signal to trigger incorrectly the TV set's color burst signal. The vertical pulses will have the effect of vertical sync signals on a TV set if reduced video amplitude is present. Most TV sets or VCRs need about 30 µs to trigger the vertical sync filter to output a vertical pulse. Thus, by modifying the vertical pulses such that even if residual pseudo-vertical pulses are of less duration than for instance 20µs, then no pseudo-vertical sync pulses are output from the vertical sync filter. Sufficient defeat of the checker pulses results if the "low" state of the checker pulse is shortened so as to cause a narrowed horizontal sync pulse not to be detected by the TV set's or VCR's sync separator. This avoids the effect of the checker pattern when playing back an illegal copy. Figure 1 shows a two step circuit for removing all the anticopy protection signals outlined above. The circuit of Figure 1 is not an embodiment of the present invention, but is provided to assist in the understanding of the invention. A video signal containing AGC, pseudo-sync, checker, vertical pulses, and narrowed horizontal-sync pulses is input on terminal 228 into circuitry 230 which is as disclosed in U.S. Patents Nos. 4,695,901 and 5,157,510 and is arranged to defeat the effects of AGC pulses and pseudo-sync pulses. The signal output from circuitry 230 is then input into an enhancement remover 234 which is arranged to defeat the checker and vertical pulses, and also defeats sync narrowing and any residual AGC pulses or pseudo-sync pulses in the horizontal blanking interval. The video signal output at terminal 236 is thus free of all effects of copy protection signals. In this circuit, the checker and vertical pulses are defeated (ideally) by replacing these pulses with pulses having an amplitude of about 20% of peak white pulses, or by adding a level shifting signal having an amplitude of about 20% of peak white. The checker pulses can also be defeated by substituting in a wide horizontal sync signal, thereby replacing the checker pulses. This defeats the checker pulses and widens the horizontal sync pulse to defeat the sync narrowing process. Finally, if the HBI (horizontal blanking interval) is replaced by new horizontal sync and color burst, then the sync narrowing and any AGC pulse and/or pseudo-sync pulse in the active field are defeated. Also in this circuit, narrowing the black level duration of the checker and vertical pulses results in a viewable copy. Also, any AGC pulses following a normal horizontal sync pulse can be defeated by adding a negative level shifting pulse to counter the elevated color burst (AGC pulse) or by sync and burst replacement. A circuit for accomplishing this is described hereinafter. In Figure 2, showing details of block 234 of Figure 1, an enhanced anticopy protected signal is input to amplifier A10 having gain K (i.e. K is equal to 2). The output of amplifier A10 is coupled to capacitor C1, diode D1, and resistor R1, which together are a DC sync restoration circuit. Resistor R2, capacitor C2, and capacitor C1 form a color subcarrier frequency notch filter so that comparator A11 can separate sync properly. Voltage reference Vb1 sets the slice point to cause comparator A11 to function as a sync separator. The output of comparator A11 is then fed to a low pass filter including components resistor R3, inductor L1 and capacitor C3 to recover a vertical rate pulse. Comparator A12 with reference input level Vb2 is a vertical sync separator. Because this video signal can be from a VCR, certain sync separators, i.e. the LM 1881, produce incorrect frame pulses with VCR outputs. So to generate a frame pulse, one shot U1 outputs a pulse that ends slightly short of six lines from the beginning of the first vertical sync pulse. One shot U2 outputs pulses having a width of about 25 µs. Then AND gate U7 logically "ands" the output of inverter U6 with that of one-shot U2 to generate a pulse occurring every two fields or each frame. Only in one field is output from U2 and U6 high. Gate U7's output is the signal (FID) gate which triggers (see Figure 3) one shot U8 having a pulse duration of 1 ½ fields. With D flip flop U9 and with the output of one shot U8 connected to the D input of flip flop U9 and vertical sync as the clock input for flip flop U9, a frame rate square wave is produced with its rising and falling edges coincident with the first vertical sync (broad) pulse of the incoming video signal. One shot U10 with counter circuit U11 and horizontal rate pulse from horizontal PLL U4 generates signals on a 10 bit address bus B10 that counts 525 states. EPROM U12 is addressed by 10 bit bus B10, and dependent on how EPROM U12 is programmed, EPROM U12's output lines carry the following signals: 1) Active field location (AF) (high states from line 22 to line 262). 2) End of field location (EOFL) (high states from line 254 to 262). In Figure 2, PLL circuit U4 and one shot U5 are a horizontal rate PLL circuit such that PLL circuit U4's output is in advance of the leading edge of video horizontal sync by about 3 µs. This is done by one shot U5, which delays PLL U4's output by about 3 µs. The output of one shot U5 is fed back to a phase detector input of PLL U4. Since the edges of both detector inputs of PLL U4 must match, PLL U4's output must then be in advance of the leading sync edge amplifier (comparator) A11. PLL U4 also ignores any pulses other than horizontal rate. Thus vertical pulses and others are ignored by PLL U4. Furthermore, one shot U3 develops a burst gate pulse (BG) of incoming video by timing off the trailing edge of sync from comparator A11. Figure 4 shows a level shifting circuit defeating the checker and vertical pulses. Amplifiers A20, A21 form a summing amplifier. Signals supplied to this summing amplifier are via resistor R100 for video, via resistor R101 for end of line pulses, and via resistor R102 for end of field pulses. By using the advance horizontal pulse (AHP) which starts at the same time as each of the checker pulses, a pulse of about 1.5 µs duration is timed off the AHP using the active field pulse AF from EPROM U12. AND gate U13 generates a logic high pulse (EOLD) at the end of each line during the active field. This pulse from gate U13 then is added to the video, which causes the checker pulses never to come down to blanking level. Instead, the checker pulses now have a minimum 20% of peak white level. This defeats the end of line checker pulses because under the circumstance of an attenuated video signal, the checker pulses do not go down in video level enough to cause a sync separator to "accidentally" trigger. Similar results are achieved for the vertical pulses where one shot U16 generates an active horizontal line pulse of about 49 µs duration (a minimum of 35 µsec) via the AHP into one shot U15 for a pulse that ends at the HBI. One shot U16 then triggers off one shot U15 (14 µsec pulse duration) to form an active line pulse. This active horizontal line pulse is and'd by gate U150 with the end of field location (EOFL) pulse via EPROM U12. The EPROM U12 EOFL pulses send a high logic signal at the end of the field during the horizontal active line via gate U150. This high logic level from gate U150 output is then added to the video signal by resistor R102 to ensure that the vertical pulses are at a minimum level of 20% of peak white. With the vertical pulses having a level of at least 20% of peak white, under attenuated video circumstances, these new end of field lines will not cause a pseudo-vertical sync. The output of amplifer A21 then contains defeat mechanisms for both checker and end of field pulses. To defeat the sync narrowing process, the output of amplifier A21 is coupled to capacitor C12, capacitor C13, diode D10, diode D11, and resistor R12, which form a DC sync tip amplifier. Voltage VD2 is set to have 0 volts DC at blanking level into amplifier A22. By using AHP again, one shot U17 and one shot U18 generate a new widened sync pulse. Components R17, C18, C3, C19, R18 and R19 are a low pass filter for finite rise time sync. Voltage Vb3 is set to establish blanking level for the "high" state of new widened sync. One shots U19 and U20 with AND gate U21 are the control logic to reinsert the new wider horizontal sync pulse during the active field. The outputs of one shots U19 and U20 are slightly delayed from U17 and U18 to accommodate the delay in the lowpass filter including components R17, C18, L3, etc. Electronic switch SW1 thus switches in the widened sync and outputs video out to amplifier A23. The right hand portion of Figure 18 within the dotted line is the sync replacement and output circuitry S50. Figure 5 shows a circuit to be used in conjunction with that of Figure 2 that replaces the checker pulses with new widened horizontal sync pulses and a new color burst to follow these new widened horizontal sync pulses. Also the vertical modification pulses are defeated by level shifting via source signal EOFD (from Figure 4) into resistors R412 and R411. Input video is fed to a chroma band pass filter including components R299, C400, L400 and C401 into burst regenerator U40 (part number CA1398) which is a burst to continuous wave subcarrier (3.58 mHz) regenerator; crystal Y40 is a 3.58 mHz crystal. The output of burst regenerator U40 is filtered via a band pass filter (3.58 mHz pass, including components R300, L401 and C402) and buffered by amplifier A40. Electronics switch SW40 gates in new color burst via one shot U43. One shot U43 is triggered by the trailing edge of the regenerated widened sync of one shot U41. One shot U40 is a 0.5µs delay to establish a horizontal sync front porch "breezeway". Regenerated sync from one shot U41 output is filtered and level shifted via components R307, L403, C404, R305, R306 and voltage V400. Amplifier A42 buffers this level shifted widened regenerated sync to sum in with color burst via resistor R304 and amplifier A43. Electronic switch SW41 gates in during the active field (via AND gate U44 and the AF source signal from EPROM U12) new widened sync and new burst during the HBI. (The new widened sync and new burst also defeats any anticopy video signals with active field AGC pulses in the HBI, i.e. raised back porch pulses). Amplifier A44 buffers and outputs new video with defeated anticopy signals including narrowed sync, checker pulses, vertical pulses. Figure 6 shows a circuit for "level shifting" by multiplying a non-zero voltage to a higher voltage. End of line defeat signal EOLD and end of field defeat signal EOFD are used for level shifting in Figure 2 generate a control voltage to raise the gain of voltage controlled amplifier (VCA) U50 (part number MC1494) during the presence of checker pulses and vertical modification pulses in the copy protected signal. The video is DC restored so that sync tip is at OV, which means low states of the checker and vertical modification pulses are above OV (typical from 0.3V to 0.5V). Components C201, R201, D10, D20, C200, R200 and A49 form this DC restored video signal. The output of VCA U50 then has a equivalent level shifted or amplified anticopy signal well above blanking level to defeat the anticopy enhancements. Amplifier A50 buffers the output signals from VCA U49 into the narrowed sync pulse defeat circuit of Figure 2. Figures 7, 8 and 9 show alternatives to defeat the checker and vertical anticopy signals by switching circuitry. Figure 7 shows that (for the DC restored video of Figure 2) during the checker and vertical pulses, control voltages coincident with these pulses switch in (under control of the EOLD and EOFD signals) a 20% of peak white signal V 10 , overriding the anticopy pulses by electronic switches respectively SW199 and SW198. The finite driving impedance of the video signal allows for this (resistor R200 providing the impedance of about 2000 ohms). The video signal is then amplified by amplifier A501, and processed by the sync replacement and output circuitry S50 of Figure 4, before being outputted at terminal 506. Figures 8 and 9 show alternative switching circuits to that of Figure 7 to defeat checker and vertical pulses. (Sync replacement and output circuitry S50 is not shown in Figures 8, 9 but is present as in Figure 7). In Figure 8, the DC restored video input signal is again applied via resistor R201 to amplifier A54, with override switches SW198, SW199 under control respectively of the EOLD and EOFD pulses switching in voltages V1, V2, each of which is a DC or DC offset AC signal greater than or equal to 20% of peak white. The circuit of Figure 9 is similar to that of Figure 8, except that switches SW198, SW199 are located serially directly in the video signal path and use conventional replacement means to overcome the checker and end of field pulses. In certain cases merely blanking the checker and EOF pulses (blanking level V 1 = V 2 = V 10 ) may be sufficient in a viewable copy to defeat the effects of checker or EOF pulses. Another circuit for defeating the checker and vertical pulses is shown in Figure 10, where since switch SW100 is of low resistance, essentially the checker and vertical modification pulses are attenuated and/or level shifted or replaced by a voltage that is the average voltage (due to switch averaging circuit 260) in the high-low states at the end of line (checker) pulses and the end of field (vertical modification) pulses. For example, if the checker and vertical modification pulses have high states of 30 IRE and low states of 0 IRE, the capacitor C1 will charge to a voltage to approximately 30IRE-0IRE 2 = 15IRE Because switch SW100 is on during the checker period at the end of the line and on during the end of field pulses due to gate U304, during these times the capacitor C1 voltage overrides the input video signal with approximately a 15 IRE level, enough to defeat the enhanced anti-copy signals. In Figure 10, enhanced anticopy video signals are fed to input amplifier A1 which outputs into sync separator circuit 258 which outputs a short duration frame pulse (i.e. 10 µs) to reset memory address counters in circuit 260. Meanwhile, composite sync (including possibly pseudo sync pulses from the prior art basic anticopy process) is fed to a horizontal phaselock loop circuit (PLL) U303. The output of PLL U303 is then a horizontal frequency pulse which starts about 2 µs before the front porch of input video. The EPROM of circuit 260 has outputs correlating to the checker and end of field pulses line locations in the TV field. One shot U100 outputs a pulse coincident with the checker location within the horizontal line, while one-shots U200 and U300 form a pulse such that the output of U300 has 3 pulse coincident with the end of field pulses within the horizontal line. The locations of checker and end of field pulses are gated through gates U202 and U203 and "or'd" via gate U304 to output pulses coincident in time with checker (EOL) and end of field pulses of the input video. Switch SW103 turns on during these coincident times to attenuate via resistor RS and average out (via capacitor C1) the enhanced copy protected signals, to output a more viewable signal from amplifier A2. Yet another defeat method (for use against the EOL and EOF pulses) is to attenuate peak active video from 100% to 80% (by about 20%), and also increase the sync amplitude (by about 50%) as shown in the waveforms of Figures 11a and 11b. This requires an increase of composite sync from 40 IRE to about 60 IRE. This can also defeat pseudo-sync pulses as in US 4,631,603 because the pseudo sync pulses therein are 40 IRE. With prolonged composite sync pulses, sync separation circuits tend to separate only the large sync pulses and ignore the smaller amplitude ones. Thus the pulse pairs (pseudo-sync plus AGC pulse) will not be sensed. Figure 11a shows the original input waveform for one video line. Figure 11b shows the video line waveform modified for both the checker and vertical modification pulses. Figure 11b shows a resultant waveform with modified sync amplitudes to be about 50% over the standard video with checker and end of field pulses. Since the composite sync signals are larger, the attenuation by the illegal copy will generally not be enough to cause the checker and end of field pulses to be of any effect when viewing an illegal copy. Because horizontal and vertical sync are modified to be much larger, the TV or VCR sync separator will not mistrigger. Figure 12 shows a circuit to provide the waveform of Figure 11 b. Enhanced anticopy protected signals are input to an amplifier A84 with gain of 0.8. These input signals are also clamped and have blanking level equal to OV. Sync separator circuit 302 outputs composite sync CS to analog switch SW210 and attenuator 300. Attenuator circuit 300 attenuates the typical logic level of composite sync (i.e. 5V peak to peak) and via offset voltage - V. Attenuator circuit 300 outputs a regenerated composite sync of 60 IRE (with 0 IRE equal to OV) to -60 IRE levels. Switch SW210 then switches in this new regenerated sync to output via amplifier A505 a waveform like that of Figure 11b. Another defeat method as shown in the circuit of Figure 13 is to track and hold the active video line to replace the checker pulse with the last value of active video before the beginning of the EOL pulses. By using from the circuit of Figure 10, A1 output and U202 output along with the circuit of Figure 13, it is possible to defeat the checkers by tracking and holding. This method is similar to reinserting a known voltage during the time of checker pulses. Since most program material is above 0 IRE (especially in NTSC where black level is 7.5 IRE), tracking and holding the video results in a level generally greater than 7.5 IRE, which is enough to defeat the checkers when this level is re-inserted in the checker location. Amplifier A90 receives input from amplifier A1 of Figure 10. Amplifier A90 has a delay of 100 ns to 200 ns (via delay lines or low pass filters) so that the pulse from gate U202 tracks and holds video 100ns to 200ns just before the checker pulses. Switch 310 is on at all times and is off during the checker pulses' times. Thus, amplifier A92 output essentially is video transparent until switch 310 turns off and capacitor C107 fills in for 2µs the last program pixel (greater than approximately 7.75 IRE) during the checker pulse time. The following describes a method and apparatus of the invention in which the basic anti-copy process signals consisting of pseudo sync and AGC (i.e. basic anticopy process) added pulses (as described above) are reduced in effectiveness, without altering these added pulses. Unlike the above described previous methods for altering the added pulses via amplitude attenuation, level shifting or pulse narrowing to offset the added pulses' effect, the present method reduces the effects of added pulses by further adding other pulses that counteract the gain reduction caused by the AGC and pseudo-sync pulses. U.S. Patent No. 4,631,603 describes how the AGC circuit in a VCR measures the incoming video signal amplitude using a sync and back porch sample. By adding extra sync pulses with a very high level back porch, gain reduction occurs. Because the AGC circuit in a VCR continuously samples the sync amplitude (via a sync sample and a back porch sample) the present method negates some of the anticopy signals by moving all the back porch levels from blanking, to below blanking level (i.e. about-20 IRE units for NTSC video). It is also possible with the present method to add extra pseudo-sync pulses in the area at the bottom of the TV field (end of field) where anticopy signals containing AGC and pseudo-sync pulses are not present. These "extra" pseudo-sync pulses are followed by pulses below blanking level. Referring to Figure 14a, the basic anti-copy protected video ("video in") is coupled to a sync separator U2 (part number LM 1881 or equivalent). The composite sync from sync separator U2 triggers the trailing edge of sync to a 3 µs one shot U3. Vertical sync from sync separator U2 triggers one shots U4 and U5 to form an active field pulse provided as an input to AND gate U1 which "ands" the active field pulse and the output of one-shot U3. The output of gate U1 is then a 3 µs backporch pulse during the active TV field. (Alternatively, one-shots U4 and U5 are not necessary and one-shot U3 output goes directly to resistor R6, eliminating U1, U4 and U5). Resistor R6 is a negative summing resistor that subtracts a level from the back porch of the video input. Input amplifier A0 buffers the video input and couples to capacitor C3, diode D1, resistor R3 and voltage Vb that forms a sync tip DC restoration circuit. The output of feedback amplifier A3 is supplied to resistor R7; this output has a lowered back porch. (See Figures 15a to 15g showing signals at various locations in Figure 14a, as labelled). The circuit of Figure 14b receives the video out 1 signal from resistor R7 of the circuit of Figure 14a and replaces the last 10 or 11 lines of each TV field with TV lines containing pseudo sync pulses paired with subsequent AGC pulses below blanking level, i.e. -10 IRE to -30 IRE. The video from the node of diode D1 of Figure 14a contains video that is DC restored to 0 volts for 0 IRE blanking level. Amplifier A2 of Figure 14b amplifies this video and couples it to horizontal lock oscillator U11 (using pin 1 of oscillator CA 31541, where the sync tip from amplifier A2 is at 7V). The output of oscillator U11 is a 32H phase lock loop and outputs a signal of about 503 KHz frequency. This 503 KHz output signal is amplified for logic levels by amplifier A2 and input to binary divider U10. Summing amplifier A4 outputs a square wave signal of approximately 2 µs on and 2 µs off of amplitude -20 IRE to -40 IRE. Voltage Vbb and resistor R9 set the proper DC offset voltage, whereas resistors R10 and R11 set the proper amplitude. In Figure 14a one-shot U6 generates an active line pulse of 32 µs duration from the beginning of the active horizontal line; one shots U7 and U8 are triggered by the vertical sync pulse to turn high during the last 11 lines of the active TV field. AND gate U9 of Figure 14b thus gates in a 4 µs period square wave of levels of -20 IRE to -40 IRE during the last 11 horizontal active lines of the TV field (where the pseudo sync and AGC pulses are not generally located). Amplifier A5 and resistor R12 output a modified anticopy signal with lowered backporch pulses and new pseudo-sync and lowered negative AGC pulses. The modified video signal provided by the circuit of Figures 14a and 14b causes the AGC amplifier in a VCR to measure incorrectly. As a result based on its measurements of the pseudo sync pulses (and with lowered back porch) paired with AGC pulses of reduced level, it appears to the VCR that a low level video signal is present, and thus the VCR increases the gain of its AGC amplifier. This offsets the reduction of the gain in the AGC VCR amplifier via the basic anticopy process. The added pseudo-sync pulses in the EOF locations each has in one embodiment at least about 2 µsec of blanking level (0 IRE) following the trailing edge of each added pseudo-sync pulse to defeat the EOF (vertical) modification. This is accomplished by a switch or waveform replacement circuit as described variously above. This is useful if the high state of the EOF modification has an amplitude greater than 10 to 20 IPE. In the absence of the blanking level under these conditions, the EOF modification effect may be reduced but the prior art basic anticopy process effect is increased, thus increasing the EOL modification and preventing defeat of the overall anticopy process. It will be appreciated that modifications to and variations in the embodiments described and illustrated may be made within the scope of the present invention as defined by the appended claims. A method and apparatus for modifying a copy protected video signal to enhance the copy protection is described and claimed in EP-A-0706740 from which the present application is divided.

Claims: A method of defeating the effects of a video anti-copy process which adds pulses to blanking intervals of a video signal, the method comprising the step of: replacing a particular portion of the video signal with a signal having an amplitude sufficiently below the blanking level of the video signal to enable a VTR's AGC amplifier to offset the gain reduction caused by said anti-copy process whereby the anti-copy process is defeated. A method as claimed in Claim 1, wherein the particular portion comprises a backporch portion of the video signal and has an amplitude at the blanking level. A method as claimed in Claim 1 or Claim 2, wherein the amplitude below the blanking level is about -20 IRE units. A method as claimed in any preceding claim, further comprising the steps of: generating a plurality of negative-going pulses; and adding the plurality of negative-going pulses to about the last ten active lines of at least some fields of the video signal. A method as claimed in Claim 4, further comprising the step of modifying a portion of the video signal following each added negative-going pulse to be at a blanking level of the video signal for a predetermined period. A method as claimed in any preceding claim, wherein the step of replacing comprises adding a negative level shifting pulse to the particular portion of the video signal. A method as claimed in any of Claims 1 to 3, wherein the step of replacing additionally comprises: determining a portion of each field of the video signal containing none of the added pulses; generating a plurality of negative-going pulses; generating a plurality of positive-going pulses having a level of about-10 to -30 IRE; and adding the pluralities of negative-going and positive-going pulses generated to the determined portion of each field. Apparatus for defeating the effects of a video copy protection process which adds pulses to blanking intervals of a video signal, the apparatus comprising: a generator for generating a signal having an amplitude sufficiently below the blanking level of the video signal to enable a VTR's AGC amplifier to offset the gain reduction caused by said anti-copy process; and means (U9,SW1) for replacing a particular portion of the video signal with said generated signal having an amplitude sufficiently below the blanking level whereby the anti-copy process is defeated. Apparatus as claimed in Claim 8, wherein said replacing means comprises: means (U2,U1) for determining a back porch portion of the video signal, the back porch having an amplitude at the blanking level of the video signal; and means (R4-R7,A3) for replacing the back porch portion with said generated signal having an amplitude below the blanking level. Apparatus as claimed in Claim 9, further comprising: a generator (U11,U10,A4) for generating a plurality of negative-going pulses; and means (SW1) for adding the plurality of negative-going pulses to about the last ten lines of at least some fields of the video signal. Apparatus as claimed in any of Claims 8 to 10, further comprising: a generator (U6) for generating a signal below the blanking level of the video signal; timing circuitry (U7,U8) for determining durations of particular portions of at least some lines of the video signal, the added pulses being present in the particular portions; and an adding circuit (U9,SW1) for adding the generated signal to the video signals at the determined duration.

Inventors: CORCORAN JEREMY J, KNOX ALISTAIR J, QUAN RONALD, RYAN JOHN O, WONFOR PETER J, CORCORAN, JEREMY, J., KNOX, ALISTAIR, J., QUAN, RONALD, RYAN, JOHN, O., WONFOR, PETER, J.
Applicants: MACROVISION CORP, MACROVISION CORPORATION
Application year: 1994
Main classification: H04N5
IPC codes: H04N5